In this proposal we explore the role of bone marrow (BM) derived endothelial progenitor cells (EPCs) in development and spread of tumors in preclinical model systems. EPCs have been shown by a number of laboratories to be essential for the formation of an intact vascular network both at the primary tumor site and the metastatic niche. in As such these cells may be important targets for therapeutic intervention if they can be destroyed selectively. The precise identification, isolation and targeting of EPCs has been hindered by the fact that many EPC markers are shared by many different cell types. Indeed, controversies still exist about the EPC phenotype, and many studies have not only questioned the relatively low contribution, but also their significance in tumors neoangiogenesis. In order to definitively address the role of EPCs in tumor angiogenesis it is necessary ideally to uniquely define this population, inhibit the activity of genes essential for their function, ablate the population specifically and show that these cells are sufficient to rescue angiogenic defects in genetic models. In this proposal, we utilize the fact that Id1 and VE-cadherin double positive cells in the adult define the EPC population. We will induce loss of Id1 function in VE-cadherin+ EPCs and determine the consequence on tumor growth. Specific ablation of the VE-cadherin Id1+ cells will be performed. Purified EPCs will be used to rescue the angiogenic deficiency in the Id1 knockout mice. Finally, peptides which home specifically to EPCs will used to deliver inhibitory oligonucleotides to EPCs in order to test the functional significance of EPC-specific genes that we identify. These studies should help clarify the role of EPCs in tumor biology and provide a basis for their targeting as a potential therapeutic strategy in the management of human cancers. PUBLIC HEALTH RELEVANCE: In this proposal we characterize a specialized group of cells which are produced in the bone marrow in response to the growth of a tumor and are essential for the formation of a functional blood vessel supply both at the primary tumor site and at sites of metastatic growth. As such these cells (called endothelial progenitor cells or EPCs) may be important targets for therapeutic intervention if they can be destroyed selectively. The precise identification, isolation and targeting of EPCs has been hindered by the fact that many of the proteins expressed on their surface used in their identification (called markers) are shared by many different cell types. In the experiments being proposed we outline a strategy to circumvent this problem by using a combination of just two markers which we show uniquely identifies the EPC population. Using a set of genetically engineered mouse strains we go on to isolate the EPC population, identify new genes that are expressed specifically within them and, finally, destroy these cells during primary and metastatic tumor growth. Having devised a strategy to inhibit in living animals the expression of any given gene within blood vessel forming cells, we propose to adapt that strategy for targeting the EPC population in preclinical mouse models of cancer and ultimately in patients. These experiments can lead to the development of targeted therapies which can reduce primary tumor growth and metastatic burden with the hope of extending the lives of patients with aggressive cancers.